1. Field of the Invention
The present invention relates generally to a solid state imaging device and a method of manufacturing the same and, more particularly, to a solid state imaging device having a photoelectric conversion portion and a micro-lens formed thereon, and a method of manufacturing the same.
2. Description of the Prior Art
Recently, in the art of solid state imaging devices, the use of micro-lenses has resulted in remarkable improvement in sensitivity.
Conventionally, there has been provided a solid state imaging devices as shown in FIG. 8A for a slid state imaging device having a micro-lens. FIG. 8A illustrates an optical system in a unit pixel of the solid state imaging device. In FIG. 8A, a reference numeral 181 donates a photoelectric conversion portion, 182 donates an interlayer insulating film, 183 indicates transfer electrodes for signal charge transfer, 184 indicates light shielding portions disposed above respective transfer electrodes, 185 represents a protective film, 186 represents a transparent resin layer or micro-lens support layer, and 187 represents a micro-lens. Shown by 188 is a vertical incident light at a lens edge, and by 189 is an oblique incident light.
As FIG. 8A shows, the solid state imaging device is such that the vertical incident light 188 is conducted by the micro-lens 187 to the photoelectric conversion portion 181 so as to provide for sensitivity improvement.
FIGS. 8B and 8C show the condition of light incidence upon the photoelectric conversion portion in the conventional solid state imaging device. In FIG. 8B, a reference numeral 190 designates an incident light beam at an ordinary diaphragm position, 191 designates an effective photoelectric conversion area, 192 designates an area of actual photoelectric conversion, and 193 designates a long wave light ray of oblique incidence reaching the proximity of a signal charge transfer region upon release of a stop. Shown by 194 is a virtual aperture stop comprised of a light-shielding film or the like.
Today, solid state imaging devices are used in a very wide range of applications including,. for example, video movies and monitoring cameras. Indeed, this versatility in use requires adaptation to all kinds of optical applications.
In various applications, it has been found that an optical change within an optical system, such as stop changing of the camera lens optics, which was not taken up as a problem in any serious way in the past, has an important bearing upon image quality, image plane brightness, and the like. In this conjunction, solid state imaging devices of the above described type have drawbacks as explained hereinbelow.
In such prior art solid state imaging device, micro-lens optics is often so designed as to best suit the condition in which the stop for the lens optics is set rather narrow. That is, a design has been considered most ideal such that vertical incident light 188 at a lens edge, as in FIG. 8A, is allowed to enter the photoelectric conversion portion without eclipse.
However, when lenses are used in a condition close to open aperture as in the case of imaging in a dark room, obliquely incident light rays as designated by reference numeral 189 in FIG. 8A will noticeably increase in their proportions to the total amount of all incident light rays. As a consequence, light rays that fail to enter the photoelectric conversion portion 181 due to an eclipse caused by a structural member peripheral to the aperture will proportionally increase, which results in a virtual decrease in optical sensitivity.
Where the solid state imaging device is a color imaging device, this involves another problem that white balance may be unfavorably affected.
In this way, conventional solid state imaging devices of the foregoing type are likely to involve image quality degradation due to changes in imaging conditions.
In such prior art solid state imaging devices, the micro-lens configuration is such that, as FIGS. 8B and 8C show, a spatial range 192 in which an incident light beam passes through a photoelectric conversion portion 191 is limited to the vicinity of the center of the photoelectric conversion portion 191. Therefore, carriers generated are locally forced into a condition close to oversaturation at a center portion of the photoelectric conversion portion, and the transition probability of electron in that portion is likely to decrease on the order of carrier diffusion time. The reason for this may be that, as a matter of basic rule, a completely depleted portion has a highest transition probability of electron.
Generally, the photoelectric conversion portion 191 itself is an N-type layer, and the peripheral part of the photoelectric conversion portion 191 is surrounded by a P-type layer. Therefore, the peripheral part of the photoelectric conversion portion 191 have a higher potential gradient and a higher transition probability of electron. As already stated, however, the prior art solid state imaging device of the above described type has a drawback that such peripheral part cannot be used for photoelectric conversion. As such, from the standpoint of sensitivity, the prior art device is far from being said to be effective.
An incident light ray which has passed through the micro-lens is basically allowed to go obliquely into the photoelectric conversion portion. Therefore, when penetration depth (5-10 .mu.m) of long-wave light rays of a visible light range into a substrate including the photoelectric conversion portion is considered, there are no few probabilities that photoelectric conversion is effected with respect to the incident light within a signal charge transfer portion adjacent to the photoelectric conversion portion or at a location very close to the signal charge transfer portion. This cannot necessarily be said to be satisfactory from the standpoint of smear inhibition.
Therefore, in order to optimize the imaging conditions, it is necessary to match the photoelectric conversion region to the micro-lens optics.
When a single lens is considered, it is generally said that assuming the lens diameter (which diameter may be considered to be the diameter of an entrance pupil) is constant, the shorter its focal length, the brighter the lens is. That is, such a lens provides higher illuminance on the image field. In more simple terms, the greater the lens curvature, the higher the sensitivity of the device is. Therefore, prior art devices are equipped with a micro-lens having a relatively large curvature.
However, if the curvature of the micro-lens is increased, at peripheral edge portions of the micro-lens, the angle of incidence of an incident light ray at a point where the light enters the lens is rendered greater relative to the tangential plane of the lens. Then, as illustrated in FIG. 10 which shows reflectance Rs of s component and reflectance Rp of p component relative to a material having an refractive index of 1.6, a usual problem is that when the angle of incidence .theta. is more than about 60.degree., the reflectance involved will be intolerably high (e.g, Rs=about 21% as in the case shown), that is, the reflection of light on the surface of the micro-lens is intolerably high. This results in decreased sensitivity.
In this way, the prior art devices involve a problem yet to be solved also with respect to the utilization of light as recited above.